Fix documentation for max_wal_size and min_wal_size

The documentation [1] says max_wal_size and min_wal_size defaults are 1GB and 80 MB respectively. However, these are configured based on the wal_segment_size and documentation is not clear about it. Attached a patch to fix the documentation.

[1] https://www.postgresql.org/docs/devel/runtime-config-wal.html

Thanks,

Sirisha